The portion of the electromagnetic spectrum with wavelengths between about 350 nm and 750 nm, which includes visible light, is useful for many purposes. For example, these wavelengths enable human vision and photosynthesis by providing enough energy to do useful work in biological systems but not so much energy as to destroy the biological systems. The sun produces vast amounts of radiation in this spectral range and the earth's atmosphere allows transmission of these wavelengths to the surface. Other natural light sources such as fire and lightning as well as man-made sources such as incandescent, fluorescent, and solid state lighting also produce electromagnetic radiation or light in this spectral range.
Lighting from the sun is arguably the “gold standard” of illumination since humans are adapted to live in solar illumination. For the sun, the quantity of light varies with time from darkness to blinding. The sun has both intense collimated light from a small area (the solar disk) and subdued diffuse light from a large area (blue sky). The spectral distribution, location, and direction of light from the sun also change in a relatively consistent way as the sun moves across the sky. These solar lighting patterns affect many biological processes such as the human wake-sleep cycle and plant and animal lifecycles, which are often influenced by solar patterns.
The dynamic variation in illumination from man-made light sources is generally much smaller than the dynamic variation in the illumination from the sun. In the cases of candles, incandescent bulbs, fluorescent tubes, and discharge lamps, the spectrum and to a large degree the amount of light provided are fixed at the time of installation. Some dimming and spectral change are possible over a limited range, but illumination from the vast majority of these lighting systems is static. In addition, where dimming or spectral modifications are possible, the dimming often decreases the energy efficiency of the light source, and spectral modifications, which are conventionally achieved with optical filters, can be very wasteful of energy. Other light qualities like location, direction, and beam divergence are also fixed in most installations. However, the value of human vision is so great that this static, inflexible illumination is acceptable in many cases.
Lighting systems and methods are desired that provide greater flexibility and dynamic qualities.